Kevlar

Composites Science and Technology 60 (2000) 421±427

E€ects of heat treatment on the mechanical properties of Kevlar-29 ®bre
C.Y. Yue*, G.X. Sui, H.C. Looi
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 11 May 1999; received in revised form 16 August 1999; accepted 22 September 1999

Abstract Thermalaging treatments under both atmosphere and vacuum environments at temperatures of 100 C to 300 C for durations from 2 to 8 h have been made on Kevlar-29 yarns. The e€ects of thermal aging on tensile strength, tensile strain and the Young's modulus of the single ®bre have been evaluated. It was established that both the tensile strength and the tensile strain decreased with increase in treatmenttemperature. For treatment at constant temperature, the heating duration did not appear to have any e€ect on the tensile strength and the tensile strain. It was also found that the Young's modulus of the single ®bre was not a€ected by heat treatment under the conditions described above. Heat treatment in vacuum did not have any e€ect on the tensile strength. # 2000 Elsevier Science Ltd. Allrights reserved.
Keywords: B. Mechanical properties; E. Heat treatment; Kevlar-29 ®bre; Tensile strength; Tensile modulus

1. Introduction Since it was launched on the market in 1972, Kevlar has been adapted to a variety of applications. It is well known as a high-strength reinforcement ®bre for plastics composites. A number of research contributions have been made on Kevlar ®bre and its composites.The researchers have noticed that the advantages of Kevlar ®bres, such as high tensile strength and high tensile modulus, have not been fully utilized in composites according to predictions based on the rule of mixtures. There are two reasons for the above observation. One was that the bonding interface between ®bre and matrix was not strong enough to transfer stress between ®bres and matrix. Theother was that the ®bres were damaged when exposed either intentionally or accidentally to thermal environments during the fabrication process or while in use, which caused degradation of the ®bre properties. The e€ects of thermal exposure on the tensile properties have been noted by several researchers. Hindeleh and Abdo [1,2] have studied the e€ect of thermal exposure on Kevlar ®bres whenexposed to a controlled atmosphere of nitrogen gas at temperatures
* Corresponding author.

above 150 C for a duration of 15 min. Parimala and Vijayan [3] have extended the study on Kevlar-49 ®bres in an uncontrolled atmosphere of air at temperatures up to 350 C for heating duration from 0.5 to 260 h. The main aim of the present study is to establish whether any thermally induced deteriorationoccurs in Kevlar-29 ®bres during the fabrication process of ®bre composites. For this purpose, e€orts were made to investigate the e€ects of thermal treatments in an atmosphere of air on the tensile strength, Young's modulus and tensile strain of Kevlar-29 ®bres. Since nitrogen has been reported to have an e€ect on Kevlar ®bre at elevated temperature [1,2], e€orts were also made to examine the e€ectsof heat treatment in vacuum. It has also been reported [3] that after 250 h and 6 h exposure to 250 and 350 C, respectively, Kevlar-49 ®bres turned brittle and crumbled with handling and were not suitable for further tensile tests. Since ®bres are commonly subjected to prolonged exposure at elevated temperatures between 100 and 300 C both during processing and in service, this temperature rangewas selected for heat treatment of Kevlar-29 ®bres. The exposure time to an elevated temperature for the ®bres during the fabrication is normally less than 15 min. However, in certain applications, the Kevlar-reinforced composite

0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00137-2

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